As a vehicle on which is mounted a driving force source such as an engine, a vehicle including a drive torque distributor is known. The drive torque distributor is capable of changing a distribution ratio of a drive torque that is given to front and rear wheels. By such a change in the distribution ratio of the drive torque, the vehicle can be switched between a two-wheel drive state in which the vehicle is driven by either the front wheels or the rear wheels, and a four-wheel drive state in which the vehicle is driven by both the front wheels and the rear wheels (see, for example, Patent Documents 1 and 2 listed below).
In the above vehicle, it is possible, according to a traveling state, to improve traveling performance of the vehicle by switching the drive state to the four-wheel drive state in which the drive torque is distributed to the front and rear wheels at a predetermined distribution ratio, or to improve a fuel consumption ratio by switching the drive state to the two-wheel drive state in which the drive torque is transmitted to either the front wheels or the rear wheels.
Also, as disclosed in Patent Document 1 below, if a four-wheel drive shift condition is established because, for example, main drive wheels slip during traveling in the two-wheel drive state, the drive torque distributor is actuated to distribute part of the drive torque to driven wheels so that the drive state is switched to the four-wheel drive state, thus traveling stability is improved.